1. Field of the Invention
The present invention relates to an optical transmitter and particularly to an optical transmitter including an external optical modulator such as Mach-Zehnder type optical modulator which modulates the light from a light source to output an optical signal with a drive voltage signal generated depending on an input signal.
2. Description of the Related Art
The LiNbO3 Mach-Zehnder type modulator used as an optical-electrical conversion element in an optical transmitter has excellent optical-electrical conversion characteristic and dynamic wavelength variation characteristic but generates change (drift of operating point) of the input/output characteristic depending on the impressed DC voltage, temperature change and aging.
Control of operating point drift is described in the Japanese publication No. Hei 02-50189.
Structure of the Japanese publication No. Hei 02-50189 is illustrated in FIG. 6.
In the method of FIG. 6, a low frequency signal is superimposed to a drive signal, the frequency element of low frequency signal included in an optical signal output from an external modulator is detected, variation of operating point is detected by comparing phase of low frequency signal output from a low frequency oscillator and the frequency element of low frequency signal detected from an optical output is controlled to be eliminated.
FIG. 7 illustrates the principle diagram of operation of the Japanese publication No. Hei 02-50189.
A drive circuit 2 amplifies an input electrical signal to the predetermined voltage amplitude.
A low frequency superimposing circuit 3 receives a low frequency signal (frequency f0) from a low frequency oscillator 8, eliminates, after superimposing of only several percents to the drive signal, this element through capacitance coupling and symmetrically superimposes low frequency modulation to the upper and lower portions of envelope of the drive signal.
The light beam emitted from a light source 1 is intensity-modulated by an optical modulator 4.
Operating point of the optical modulator is determined by a bias control circuit 9.
A part of the modulator output is branched by an optical branch 5, converted to an electrical signal by a light receiving element 6, detected in its only low frequency element superimposed by a band-pass filter 71 and is then input to a synchronous detecting circuit 73.
A low frequency signal from the low frequency oscillator 8 is input to another input of the synchronous detecting circuit 73 for phase comparison with the detected low frequency element.
Amplitude of the detected low frequency element is minimized ((a) of FIG. 7) when the drive signal is located on the curve between the desired valley and mountain of the opto-electric conversion characteristic of optical modulator 4.
Polarity of detected low frequency element is different depending on the optical-electrical conversion characteristic which shifts to a higher voltage side (c of FIG. 7) from the optimum point (a) or to a lower voltage side (b of FIG. 7) by the drift.
Therefore, the signal of different polarity can be obtained depending on the shifting direction of an output compared in phase in the synchronous detecting circuit 73.
The bias can be set to the optimum point (a) of FIG. 7 by integrating such signal with a low-pass filter (LPF) 74 and then feeding back it to add the bias voltage to the modulator using the bias control circuit 9.
In this method, the operating point of the optical-electrical conversion characteristic is fixed to the center.
However, on the occasion of transmitting the signal to the transmission line in which the erbium-doped fiber optical amplifier is connected in many stages, more excellent sensitivity characteristic can be obtained by shifting the operating point from the center and it is desirable that the operating point can be set to the desired position.
Regarding this point, the Japanese publication No. Hei 08-548366 discloses the structure that operating point can be set to the desired area by adding an operating point shift circuit.
The Japanese publication No. Hei 08-548366 executes phase comparison by adjusting polarity and amplitude of a part of the output of low frequency oscillator, while the frequency element of low frequency signal included in the optical signal output from external modulator is detected and phase of low frequency signal output from the low frequency oscillator is compared in the Japanese publication No. Hei 02-50189.
As the operation, control is performed so that the frequency element of low frequency signal detected from an optical output and low frequency signal to be added are removed.
Therefore the operating point can be fixed to the point including the offset at the center of the optical-electrical conversion characteristic.
However, since power element of low frequency signal detected from optical output depends on the modulator optical output, when output optical power of modulator varies, for example, the low frequency signal added to generate offset does not change even when output amplitude of frequency element of low frequency signal detected changes and as a result the offset amount of modulator may probably be changed.
An output power of modulator will change when optical output of light source changes or when operation loss of modulator changes by aging characteristic and change of environmental temperature and since such change may be generated in the practical operation, a certain kind of measure has been requested.